Hard dime sized dark spot in egg

Hi. I am fairly new to owning chickens. My chickens have been laying all summer now. One day I found an egg in a spot other than it's nesting box. I picked it up and it felt a little heavier than normal and a shook the egg and I felt something move inside. We candled it and saw a dime sized dark spot in the egg. We didn't know what to do so we marked the egg and put it in the nesting box. Do you think it could have been a growing chick inside and why would it not be layed in the nesting box? Do you think it died? I hope someone can answer my concerns. Thanks!

Greetings from Kansas, DanaT, and ! Pleased you joined us! If you have a rooster it could be a fertile egg but if if the hen wasn't incubating it, it wouldn't have a baby chick inside. Do you have any idea how old it might be? My fear is that it was laid and then left and it might be rotten - handle with care!!!